[WIP] About ghost page

New method: Add a grunt task that builds an about ghost page that is particular to that release and puts it in the right place for the client.

1. [x] query GH API for top contributors between releases
2. [x] Create hbs partial for user list from 1 @ `core/client/templates/-contributors.hbs`
3. [x] Download avatar for each user, and move to `client/assets/img/contributors/<name>`
4. [ ] Insert ghost config (DB, Version, Environment, Mail) (waiting on #3619 / @halfdan? :D)
Talked with Hannah on this, she wanted it to be configurable, so that she can do custom "these are the people that count for this version" stuff.